This invention relates in general to compressors and in particular to a new and useful apparatus and method for controlling an operation of a turbocompressor so as to prevent pumping or surging.
In turbocompressors, surging or pumping is a process in which feed medium flows in surges from the compression side back to the suctions side. Pumping sets in when the pressure ratio between end pressure and suction pressure is too high or the throughput is too low. A so-called pumping limit line on a curve which separates the stable working range from the instable range in which pumping occurs can be defined in the pressure throughput characteristic field. To control the compressor so as to avoid pumping, a blow-off line of the compressor is preset in the characteristic field which runs parallel to the pumping limit line at a safety distance. If the momentary working point of the compressor approaches the blow-off line, a blow-off or recycle valve branched of the compressor outlet line is opened to lower the end pressure or increase the throughput. Such a pumping limit control is known from the article by Blotenberg "Turbolog- The Electronic Control System for GHH Turbomachines" in Nachrichten fur den Maschinenbau (News for Machine Builders) No. 3, May `82 as well as from German AS No. 26 23 899 and the U.S. Pat. Nos. 4,142,838 and 4,386,142.
The procedure in such pumping limit controls has so far been to measure the pumping limit of the compressor when starting initially and, based on this measurement, to preset the blow-off line at a preselected safety distance from the pumping limit line. Therefore, the shape of the blow-off line is based on the shape of the pumping limit line measured at acceptance or commissioning. Usually, however, acceptance tests are run under different marginal conditions than prevail in operation in practice, e.g. regarding the dynamics of working point shifts in the characteristic field. If the working point shifts quickly in the direction towards the instable range, pumping surge will occurr in some compressors sooner than when the working point changes slowly. This means that a pumping limit line measured under acceptance conditions with slow working point changes may be too far to the left in the characteristic field for operation in practice. Furthermore, the actual pumping limit line may vary as the hours of compressor operation increase, e.g. by contamination, zero shifting of a transducer or drift of the measuring range. Different feed medium compositions also may have an effect on the location of the pumping limit line.
All these uncertanties and inaccuracies must be taken into account when determining the safety distance between the blow-off line and the pumping limit line. This often leads to an unnecessarily great safety distance, i.e. to an unnecessarily frequent response of the pumping limit control and opening of the blow-off valve without the danger of pumping being present. This causes undesired blow-off losses.
On the other hand, if the safety distance is made too narrow, it may happen in later operation that the blow-off line runs too close to the pumping limit line and that the pumping limit control does not respond in time to prevent, by opening the blow-off valve, that the pumping limit is reached and frequent pumping surges occur.